Your tuner should have calibrated a built in safety net that accounts for small swings in ethanol percentages throughout the year. The net is weaved in threads of the timing tables, forward feeds in boost control strategy, AFR... The ECU is ridiculously complicated; literally hundreds of tables. A calibration for a stock turbo car on 91/93 is close to hundred tables touched, if not more. E85 adds more, and big turbo adding more to touched on top of that. Of course, once the tuner has his tuning strategy, he's not making hundreds of adjustments on your specific car on every revision, but they are checked. Anyway, my point is that E25-E35 swings aren't going to damage your car if your tuner has done his job, so as long as you know your basic E85 stations percentage you're fine without the gauges. If you want to be a little more thorough, check bi-annually, in Spring and Winter. There's a document out on the internet that lists, by region, when summer and winter blends are delivered (some hotter climate regions do not change). Typically changes are in March-April (E80-E85) and October-November (E70). Only if you are determined to have maximum power at all times should you worry about the content analyzer.